The battery field is historically old. Secondary (rechargeable), multi-cell batteries are divided broadly into two types of configurations, i.e., (1) parallel plate batteries, such as are commonly used in automobiles for starting and lighting; and (2) bipolar batteries.
Each cell of a parallel plate battery comprises positive and negative electrodes, a porous separator to prevent the electrodes of opposite polarity from touching, and electrolyte. Each cell usually contains several positive plates connected to one another by a conductive strap, and several positive and negative plates are alternated.
The typical parallel plate battery comprises many individual cells. For example, the battery in automobiles typically have six cells. The cells are connected in series, such that the positive electrode from one cell connects through another conductive metal strap, to the negative electrode of the adjacent cell. The negative electrode from one "end" cell of the battery is the negative battery post, and the positive electrode from the other end cell is the positive battery post. Current flows through the battery from the negative post to the negative electrode of one end cell, through the electrolyte in that cell to the positive electrode, through the strap from that positive electrode to the negative electrode of the adjacent cell, and so on, exiting at the positive electrode of the other end cell at the end post. The battery voltage (potential) is the sum of the voltages from each individual cell.
The parallel plate battery has been used successfully for decades. It has the chief advantage of preventing leakage of electrolyte from one cell to the other. Its disadvantages, especially in respect to high specific energy and specific power, are:
1. Extra weight associated with the intercell straps; PA1 2. Extra weight associated with the electrode grids, or the "skeleton" of the electrodes; PA1 3. Comparatively large distances between the positive and negative electrodes within a given cell, which results in ohmic losses (i.e., losses of potential or voltage) as the current flows through the electrolyte in the cell; PA1 4. Ohmic losses as the current travels through the electrode grids and intercell straps; PA1 5. For some battery systems, excess electrolyte (which adds weight); PA1 6. For some battery systems, uneven current distribution (i.e., more current per area flowing in the middle of the electrode plates than at the edges, or vide versa, a consequence of the geometry of the parallel plate cell design, which reduces the number of times the battery can be recharged).
On the other hand, a bipolar battery typically comprises a stack of bipolar electrodes, each prevented from touching the adjacent electrodes by a separator. Each separator holds electrolyte. Around the edges of the bipolar electrode, a seal is used to prevent the electrolyte from leaving the cell and to prevent shorts. The bipolar electrode traditionally comprises a thin, electronically conductive material known as the bipolar plate or bipolar electrode substrate, with the positive electrode active material applied to one side, and the negative electrode active material applied to the opposite side. On the ends of the bipolar battery are placed single-sided (monopolar) electrodes.
In general, the bipolar battery has several advantages over the parallel-plate battery design. First, the cumulative weight of several components is much less. Second, the efficiency of each cell is typically much higher, since it is possible to maintain a very small intercell gap (distance between facing electrodes) thereby minimizing internal losses. Well designed bipolar batteries offer peak specific powers which are often an order of magnitude (or more) higher than those of parallel plate batteries, and specific energies (W-hr/kg) can be higher for bipolar batteries.
In the early part of this century, battery manufacturers elected to use the parallel plate design as opposed to the bipolar design for greater reliability. Since then, periodic efforts have been made to develop secondary (rechargeable) bipolar batteries, mostly in response to certain applications particularly where high specific power was of principal concern. However, none of these efforts resulted in commercialization. There have been two apparently successful commercial (non-rechargeable) bipolar batteries, one by Polaroid (a four cell LeClanche battery for cameras) and one by Gould (in conjunction with Polaroid), a two cell lithium-manganese dioxide battery.
In the 1980's the Strategic Defense Initiative Organization was formed and began efforts to develop batteries which could deliver huge amounts of energy as sub-second pulses, to power certain defense systems (such as lasers). LaFollette and Bennion developed the design principles for bipolar lead acid batteries which provide high specific power. During recent years, other efforts have been made at high specific power bipolar battery development. The renewed electric vehicle development of the past four years, funded largely by the three U.S. automakers and the U.S. government, has also increased activity in bipolar battery development.
Notwithstanding bipolar battery efforts prior to the present invention, development of a rechargeable lightweight bipolar battery capable of storing large amounts of energy and of delivering energy at very high power levels has not been achieved.